Counterfeiting of consumer goods, paper currencies, financial documents and identification cards is countered by a large variety of optical security measures designed to deter and defeat this illicit activity. Hidden image optical devices, where the images become visible only under polarized light, are particularly effective optical security devices for anti-counterfeiting and brand protection applications. Hidden image technology has been widely adopted by the optical security industry for document authentication, anti-counterfeiting measures, and brand protection applications. This optical effect is alternatively referred to as an “invisible image” or a “latent image” or a “covert image”. Most of the hidden image technologies fall into two main categories: fluorescence-based, where the images become visible under UV or blue light illumination, and optical-anisotropy-based, where the images become visible under polarized light illumination. Hidden images of the latter technology are invisible when viewed with unpolarized light but can be seen when observed though simple sheet polarizers. For hidden image technologies based on optical anisotropy, namely, phase retardation, liquid crystal polymer has been chosen as a preferred material because it possesses large anisotropy and can be modified relatively easy using standard industrial processes.
Schadt et al. discloses in U.S. Pat. No. 7,679,701 a transmissive optical element having a substrate, a linear polarizer, a layer of photo-oriented polymer network (PPN, -LPP) and a nematic liquid crystal (NLC) polymer layer where the local orientation of the NLC molecules is determined by a patterned LPP layer. In selected discrete domains the orientation of the NLC molecules (equivalent to their optical axis) is different than that of the background area therefore an “image of orientation” is formed. However, this image remains hidden unless another polarizer is placed on top of the optical element. Areas where molecules are aligned at ±45° to the optical axis of the polarizer will have maximum or minimum transmission. Areas where molecules are aligned in other directions will appear to have different shades of gray. This particular device relies on a photo-oriented LLP polymer network material to create different optical axes of orientation. The LLP material and the optical alignment technique are non-standard and the latter requires specially designed tools and, therefore, the end product is expensive.
In U.S. Pat. No. 8,885,121 Quintana Arregui et al. teaches a device and a method of producing it for document security applications having multiple hidden images. The invention is based on a patterned, dichroic dye doped nematic liquid crystal polymer (NLCP) layer where the molecules in each patterned domain are aligned in a “twisted-nematic” (TN) configuration. The images are invisible when viewed with unpolarized light but each TN domain acquires a different gray shade when viewed through a polarizer. One advantage of this device is that the viewer can observe a different image from each side of the device. In addition, only one polarizer is needed for viewing the hidden image. However, to produce this device, one needs to employ a confinement plate, a non-standard manufacturing component, as an intermediate alignment substrate, which complicates its production. Additional concerns are the complexity of the process and the cost of materials.
In another class of hidden image technologies based on optical anisotropy, an anisotropic pattern is selectively induced in otherwise isotropic polymer layer. A number of processes can be used to impart a preferred orientation to randomly oriented anisotropic molecules in selected areas in such materials. Borovkov et al. discloses in U.S. Pat. No. 8,227,024, an isotropic polymer layer which is processed thermal-mechanically by generating micro-lines in selected areas. Due to mechanical stress, the above process induces locally a large optical anisotropy in a specific direction determined by the fabrication process. Differently processed domains have optical axes in different directions while the unprocessed background remains isotropic. This method provides a reliable way of producing large batch of repeatable hidden image labels. However, this process is relatively slow and has a low throughput since it relies on a digital plotting mechanism. In addition, the resolution of the hidden images is limited by the size of the plotting needles.
Karasev, et al. discloses in U.S. Pat. No. 6,124,970 a latent image device that is also based on variations in the direction of anisotropy in a polymer layer. The anisotropy is induced by exposing a special polymer and dopants to actinic radiation. This device offers a latent image with good contrast and good mechanical properties. However, its production process is complex: it uses a polarized light source to alter the anisotropy of the polymer, requires a lengthy pre-soaking of the polymer in a dopant solution and a time-consuming developing process, all of which make this device costly.
The current invention discloses an optical security device with a hidden image feature, comprising a single-layer nematic liquid crystal polymer (NLCP) having a uniform optical axis throughout said device, where different image domains differ in their retardation values. The current invention, furthermore, discloses a simple fabrication method which is roll-to-roll compatible, thus enabling a high throughput manufacturing process and a low-cost device, solving many of the difficulties associated with devices disclosed in the prior art.
The hidden image optical effect cannot be reproduced by counterfeiters employing standard reproduction techniques or using non-NLCP materials. Users of optical security devices prefer labels with multiple security levels for which NLCP is particularly suitable. Since NLCP are transparent in the visible range, hidden image devices can be overlaid on holographic based, or any other metallic based security devices, without obscuring the underlying information or optical effects.